In works of art and craft, the colors that the works have are one of the most important characteristics. For example, a painting or a pot itself does not emit light. Thus, in an art museum or the like, the illumination at a time of appreciating an article on exhibition is as important in meaning as the article itself. The reason for this is that a person who appreciates the article on exhibition observes that portion of the visible light radiated from an illumination light source that is reflected by the surface of the article. Even if an artist expresses very beautiful colors, unless the light from the light source that is irradiated on the article on exhibition includes emission light components corresponding to specific colors, the person appreciating the article can only observe the article with tonality which is dark and poor in color sensation.
The most desirable light source for this purpose of illumination is sunlight. As sunlight is composed of consecutive wavelength components of light, the sunlight includes, substantially equally, all light components of visible light wavelengths from 400 nm to 780 nm, and can reproduce all colors existing in nature as original colors inherent to substances in nature. However, no matter how excellent the sunlight is as the light source, precious works of art, such as paintings, are not appreciated in an outdoor bright space, while such works of art are directly exposed to sunlight. The reason why the works of art are stored in a specific place such as an art museum and are appreciated is, in part, because it is necessary to protect them from accidents such as by the weather and sealing. However, a more important reason is that it is necessary to protect the works of art from a great quantity of irradiation.
This is because sunlight includes the visible light of all wavelengths, and also includes emission light components other than visible light, such as ultraviolet and infrared. In particular, since ultraviolet is stronger in energy than visible light, direct exposure to sunlight promotes fading and embrittlement of historic paintings, etc. Thus, an artificial light source is needed. However, the artificial light source is required to be able to reproduce sunlight as correctly as possible, in addition to having such convenient features of artificial light that the amount of light is adjustable and the amount of ultraviolet is reduced as much as possible.
On the other hand, in recent years, as an artificial light source, attention has been paid to a light source using an LED (light-emitting diode) from the standpoint of energy saving and reduction in emission of carbon dioxide. Compared to a conventional incandescent bulb using a tungsten filament, the LED has a long lifetime and can achieve energy saving. Because of the convenience of the LED, LED illumination is rapidly gaining in popularity in the market. In many types of LED illumination in the early stage, white light was obtained by combining a blue light emitting LED and a yellow light emitting phosphor, and only unnatural white which lacks in warmth could be reproduced. However, with remarkable enhancement in performance in step with the increasing popularity of LED products in the market, various improvements have been made to combinations of LEDs and phosphors. As a result, some white light sources that can reproduce sunlight have been developed.
Patent document 1 discloses an invention relating to a white light source having the same light emission spectrum as sunlight. Sunlights with different color temperatures are reproduced with blackbody radiation spectra of the same color temperatures. In this invention, a white light source can be obtained which reproduces light close to sunlight of various color temperatures which vary with time, not only with respect to apparent white light but also with respect to the spectrum shape. Patent document 2 discloses an invention relating to an illumination system using a white light source, and relates to office illumination or the like, with the object of illumination being mainly a human or the like. This system can adjust color temperature or illuminance of indoor light, while detecting a variation of outdoor light. White light illumination corresponding to variations due to physiological phenomena of humans and seasons can be obtained. In addition, patent document 3 discloses an invention relating to an artificial sunlight system in which a plural of light-emitting diode modules with different color temperatures are combined. This system can reproduce variations of color temperatures of sunlight which is radiated on places of different latitudes and longitudes on the earth.